1. Field of the Invention
The present invention relates to a zoom lens for projection used as a projection lens of a projector apparatus or the like. Particularly, the present invention relates to a zoom lens for projection appropriate for a so-called front-type projector apparatus, which magnifies and projects an original image displayed on a light valve, such as a liquid crystal display device, onto a screen, and to a projection-type display apparatus on which the zoom lens for projection is mounted.
2. Description of the Related Art
A so-called front-projection-type projector apparatus, which projects images onto a screen in front of the apparatus, is becoming generally used for school education, company training, presentation, and the like. Further, as the market of projectors matured, the variety of projectors increased, and various projectors, such as a large projector for a large theater screen and a very small projector for a mobile phone, became used.
Meanwhile, in the market of data projection apparatuses, which is the oldest projection apparatus market, a demand for small low-cost projectors having high resolving power that can project high light-intensity images is strong. To satisfy such demand, smaller light valves, such as a liquid crystal display panel (DMD and the like are included, but hereinafter, simply referred to as a liquid crystal display panel or the like), having high resolving power are being developed. Further, a request for development of zoom lenses corresponding to such light valves is strong.
In data projection, projection of data in a light room environment is often requested. Therefore, it is essential that the optical system of a data projection apparatus has a small F-number. However, since display areas of small liquid crystal display panels are small, when rays of light similar to those passing through conventional liquid crystal display panels are tried to be passed through the small display areas, it is necessary to increase the incident angle of rays entering the liquid crystal display panel or the like, or to use a high luminance light source including a light output unit smaller than a conventional light output unit, as well as lowering the F-number of the optical system than a conventional optical system.
However, since further reduction in the size of a light output unit in a high luminance light source tends to be difficult, the aforementioned measures are difficult to be taken. Further, when the size of the liquid crystal display panel or the like is tried to be reduced, the size of a pixel becomes smaller, and it becomes necessary to cope with a higher spatial frequency than a conventional liquid crystal display panel or the like. Consequently, the cost increases.
In this circumstance, zoom lenses for projection that can satisfy the demand for a fast small-size low-cost zoom lens to some extent are known (please refer to U.S. Pat. No. 6,204,976 (Patent Document 1), Japanese Unexamined Patent Publication No. 2006-039033 (Patent Document 2), Japanese Unexamined Patent Publication No. 2006-065026 (Patent Document 3), and Japanese Unexamined Patent Publication No. 2007-206331 (Patent Document 4)). These zoom lenses for projection are so-called two-group-movable zoom lenses, which perform zooming by moving two lens groups. Further, each entire lens system is composed of 6 to 13 lenses.
Patent Documents 1, 3 and 4 disclose zoom lenses for projection with F-numbers exceeding 1.7. Further, Patent Documents 2 through 4 disclose zoom lenses for projection including an aspheric lens, or two or three aspheric lenses.
When the size of the liquid crystal display panel or the like is reduced as described above, the cost of the liquid crystal display panel or the like can be reduced. At the same time, if the size of an illumination system and the size of a lens for projection are reduced, the cost is further reducible.
However, when the intensity of light is tried to be maintained while the size of the liquid crystal display panel or the like is reduced, it is necessary to lower the F-number of the lens for projection (fast lens), and to increase the incident angle of rays entering the liquid crystal display panel or the like or to mount a high luminance light source including a small light output unit, as described above. Therefore, the cost greatly increases.
Further, when the number of movable lens groups for changing magnification is increased to three or greater to lower the F-number of the zoom lens, the cost for production of the lens system greatly increases.
To reduce the cost for production of zoom lenses, the number of lenses in the entire system should be smaller, and the outer diameter of each lens should be smaller. Further, the relative illumination of the lens should be minimized to reduce the outer diameter of the lens, or the like.
However, when the relative luminance is reduced, light energy corresponding to the reduced relative luminance is consumed to increase the temperature of a lens barrel surrounding the lens. Especially, when an aspheric lens made of plastic (the cost is lower than glass) is arranged on the reduction side of a stop, the fluctuation of an image plane and the fluctuation of field curvature or the like caused by the change in temperature become large. Further, a difference between a temperature around the aspheric lens made of plastic arranged on the reduction side of the stop and a temperature around an aspheric lens made of plastic arranged on the magnification side of the stop becomes too large. Further, time periods for returning to steady temperature differ from each other between the two aspheric lenses, which are arranged apart from each other. Therefore, it is difficult to accurately offset the influences of the change in temperature on the two aspheric lenses made of plastic each other.